Method and apparatus for ascertaining time of explosion



Nov. 24, 1953 D. M. M FARLAND 2,660,716

METHOD AND APPARATUS FOR ASCERTAINING TIME OF EXPLOSION Filed May a. 1950 2 Sheets-Sheet 1 osCiLLosRAP'rT w -IMIIE FIG.I

DAVID M; MOFARLA ND, INVENTOR.

Nov. 24, 1953 D. MPM FARLAND 2,660,716

METHOD AND APPARATUS FOR ASCERTAINING TIME OF EXPLOSION Filed May 6, 1950 2 Sheets-$heet 2 SOURCE OF 63 CURRENT 65 DAVID M. McFARLAND, INVENTOR.

?atented Nov. 2 5, i953 ATENT orrics METHOD AND APPARATUS FOR ASCERTAIN- ING TIME OF EXPLOSION Application May 6, 1950, Serial No. 160,518

'7 Claims.

An object of the present invention is the provision of an improved method for ascertaining the time at which an explosion occurs.

Another object of the present invention is the provision of an improved method for ascertaining the time of explosion of an electric explosion initiator.

Still another object of the present invention is the provision of an improved method for ascertaining the time of explosion of an explosive charge initiated by an electric explosion initiator.

A further object of the invention is the provision of an improved method for ascertaining the instant of firing of an explosive contained in the bore hole and initiated by an electric blasting cap.

Still further objects of the invention are the provision of apparatuses to accomplish the foregoing objects.

Other objects of the invention will be apparent from the following description.

In some explosive operations, and particularly in geophysical prospecting with the seismograph, it is desirable to ascertain the time of firing of an explosive charge precisely. Often the possible error should be limited to a fraction of a millisecond.

Several methods have been proposed for making such firing time determinations. Most of these depend upon ascertaining the time of firing or the electric blasting cap used to initiate the explosive charge. One such method involves the winding of a current carrying wire around the cap used, or around another cap fired in series with the cap used, and then determining the time of breaking of the electric current passing through the wire. Unfortunately it has been found that such a wire does not always break as instantaneously as may be desired. Furthermore, it is awkward to provide a current-carrying wire around the cap used, and, when a current-carrying wire is wrapped around a cap in series with the cap used, error is likely to be introduced due to the fact that electric blasting caps in series often do not detonate with sufficient simultaneity.

Probably the most common method of determining the time of firing or" a seismograph charge initiated by an electric blasting cap involves a determination of the time the bridge wire in the firing circuit of the cap breaks due to the explosion of the cap. However, from numerous tests with the cathode ray oscillograph it has become evident that the results obtained by this method are subject to appreciable variation. For one thing any inductance the firing circuit is apt to provoke an arc of unknown duration when the circuit is disrupted. Moreover, since explosions produce ionized gases, conditions in the bore hole are conducive to the preservation of current flow in the firing circuit after the bridge wire of the blasting cap has been broken.

It has been found, in accordance with the present invention, that, if an electric potential is impressed between the firing circuit of an electric explosion initiator and a point normally insulated from the firing circuit but in the locus of explosion, when explosion occurs a current will pass by virtue of the impressed potential. Thus ascertaining the time of passage of a current ascertains the time of explosion. It is postulated that the current is carried by ionized gases formed in the explosion.

In the performance of the invention, the point normally insulated from the firing circuit may be any point positioned in the locus of explosion so that it comes into conductive relation with the firing circuit. Thus, with a common electric blasting cap having a metal shell insulated from its firing circuit, a potential may be impressed between the shell and the firing circuit; or Where, as is usually the case, the explosion, the time of which is to be ascertained, takes place in the ground, the potential may be impressed between the firing circuit and ground.

The particular device used for determining the time of passage of a current between the firing circuit and a point in the locus or the explosion is no part of the present invention. Any device which will ascertain the time of commencement of an electric current may be used. An oscillograph, such as the cathode ray oscillograph or the Duddell oscillograph, is usually satisfactory. The cathode ray oscillograph appears to be the instrument most advantageously used and will be the instrument referred to as illustrative in the following description.

The potential impressed between the firing circuit and a point in the locus of explosion may be the same potential used to pass a current through the firing circuit to fire the cap. When this is done, however, it is desirable to include a resistance in the side of the cap firing circuit which is common in potential to the point external to the firing circuit in the locus of the explosion. If no such resistance is present the potential impressed external to the firing circuit may be short circuited by a continuance of a fiow of current through the firing circuit after explosion has taken place. Such a continuance of flow of current might arise, for example, due to reestablishment of contact between leg wires in the course of the explosive action or due to passage of current through the ionized gases resulting in the course of the explosion. While the resistance used should be sufiicient to divert current to the ground circuit it should not be so large that it prevents the passage of an adequate firing current.

The invention will be further and more specifically described by reference to the drawings in which:

Figure 1 is a side elevation, partly in section, of an electric blasting cap especially adapted for use in the present invention and (on a comparatively reduced scale) an electrical circuit which will bring about the accomplishment of the method of the invention;

Figure 2 represents a side sectional View of a bore hole containing an explosive charge and a blasting cap which is connected to a circuit such that the method of the present invention can be accomplished;

Figure 3 is a view similar to Figure 2 showing a modified bore hole and a modified circuit; and

Figure 4 is a diagrammatic representation of a multiple switch blasting machine suitable for use in the circuit of Figure 3.

As shown in Figure 1 H represents a metal shell into the bottom of which are. pressed base charge It, primary detonator charge i5, and flash charge H. The particular composition and number of these explosive charges is no part of the present invention, but for purpose of illustration it may be stated that base charge I3 may be tetryl, the primary detonator charge may be nitromannite, and the flash. charge I i. may be a mixture of nitromannite and diazo di-nitrophenol. [9 represents generally a conventional electric match made up of insulatingv sheet 2i on each side of. which are aflixed metal terminal sheets 23 and 25. 21. represents a match head made, up of an ignition composition, for example, a nitrocellulose lacquer containing copper acetylide, surrounding the lower portion of terminal sheets 23. and 25 and a bridge wire connecting them (not shown). 29 representsa. sealing plug which may, for example, be made of rub-. ber. Connected to match head l9. and extending through, plug 29. are leg wires SI and 33. covered by, insulation and. 3.! respectively. Also ex-- tending throughplug, 29 is wire 39, covered with insulation 43. It will be noted that wire 39' is bent around the bottom of plug 29 soasto insure itsbeing in contact withshell M.

The electric blasting cap just. describedis unusual inthe inclusion of third wire 43. It is more fully described in copending application, Serial No. 115,439, filed September 13, 1949, forElectric- Explosion Initiator, of which thisapplication is a continuation in part.

Leg wire 31 is connected to one pole of a conventional, blasting machine: 43 and. leg,- wire 33 is connectedto theother pole ofblastingmachine 4'3; Wire isconnected to the same pole-ot'the blasting machine as is leg Wire 33 and oonnectstheblasting machine to an oscillograph. The oscillographis in turn connectedto abattery ll which initsturn isconnected to the third wire 3.9 of theelectric blasting cap. Thus, by means of battery 41 a. potential is impressed between shell it and thefiring circuit of the blasting capthroughthe oscillograph. When blasting machine is operated and a, current. passesthrough leg wires 3i and 33 to match it, the blasting cap fires. The

4 ionized gases so formed permit the passage of a current between the firing circuit and third wire 39. The making of this circuit is recorded on the oscillograph and indicates the time of firing of the blasting cap.

It is not necessary that third Wire 39 be in electrical contact with shell II. It may simply terminate inside the initiator so as to provide a pole for passage of current through the ionized atmosphere resulting after the cap has fired. However, when wire 39 is connected with shell I l as is preferred, a larger contact is, of course, provided.

The invention may also be performed with ord-inary electric blasting caps, and an arrangement for doing this is shown in Figure 2 wherein 49 represents a bore. hole, 5! represents an explosive charge, 53 represents an electric blasting cap, 55 and 51 represent the two leg Wires of electric blasting cap 53, and 59 represents stemming material loaded above charge 5!. Many bore holes used in seismograph blasting have metal casings extending part or all of their length. Such a casing is shown at 5!. Blasting cap 53 is connected by its leg wires 55 and 5'! to a source of current for firing it, and another source of current is connected to casing 5| of bore hole 49 by means of wire 63. The second source of current is connected by Wire 65 through an oscillograph and wire 51: to the firing circuit of the blasting cap so that the. second source of current and the oscillograph are serially connected between casingt! andthe firing circuit.

In operation of; the circuit just described the second source of current impresses a potential I through the oscillograph between ground and the firing circuit of the blasting cap. The first source of current fires the blasting cap. When the 05-. cillograph registers the startingtime of passage of a current the time of explosion has beenascertained.

Whileboth sources of current may be applied simultaneously, it is usually desirable to impress the potential through the oscillograph, a, short time before closing the firing circuit ofthe blasting cap. However, to avoid danger from, accidental firing of the electric blasting cap in the eventitshould have haditsfiring circuit grounded to-its shell. during manufacture, it is not usually desirable to impress the potential: between the firing circuit and grounduntil all steps prior to-firing have been completed As has-,beerlpstatedabove, the same source of current, may be used; both to fire the cap and to register; the time of firing thecap. Figure 4 illustrates a manner in which a multipleswitch blasting machine may be, used both as a single source of; current and, as a means for impressing the potential; between ground and thefiring ciri just-pr oroth ime o firmei ith as ing cap. A number of elements Figure 3 are similar to elements appearing in Figure 2 are represented by'primednumbers corresponding to numbers used in Figure 2. In the ground is borehole; 49 containingan explosive charge 5! in which is placed an electric blasting cap 53 provided with leg wires 55 and; 5"! leading out of; bore hole 49. Stemming 59' is packed above charge, 5} in bore hole 4 9. Leg wire 55. leads to resistance'iawhich is connected in series withmultipleswitchblasting machine 11 through wire 15.

Multiple switch blasting machine H is dierammat c y shown. n er de E ur e. 4 and is; fully described; in- U. S. Patent 2,081,633, issued May 25, 1937 for Method of and Apparatus for Firing Explosive. It is operated by depressing handle I00. Rack bar I05 when thrust downwardly by means of handle I06 imparts a quick spin through suitable gears (not shown) to the armature of a generator represented diagrammatically at 8|. After the generator has picked up speed and built up electric energy, the lower end of the rack bar acts upon a cam lug 09 to close an electric circuit by bringing the contact foot H into engagement with contact strip III. Contact strip I II is connected by a conductor H! with binding post H8, and contact foot I I0 is connected by conductor H9 to the generator windings indicated at 12!, these latter being connected by a conductor I22 with binding post I23.

After contact foot I I0 has been brought into engagement with contact strip H I, rack bar I95 continues its downward movement and through cam lug H2 thrusts contact foot H3 against a contact strip H4 thereby closing a second circuit. Contact strip H4 is connected by a conductor H with binding post H6, and contact foot H3 is connected by conductor I20 through the generator windings I2I and conductor I22 with binding post I23.

Referring again to Figure 3 it is seen that wire 19 is connected to binding post H6 of multiple switch blasting machine 11. Leg wire 51' is connected to common terminal I23 of blasting machine II. Terminal H8 is connected to ground through an oscillograph by means of wires 83 and 84. Thus, as blasting machine I1 is operated a potential is first impressed between the firing circuit of the blasting cap 53 through the ground and the oscillograph; then, while that potential is still applied, a current is passed through the blasting cap firing circuit (and through resistance 15) firing the cap. As the explosion takes place, the potential impressed between the cap firing circuit and ground causes the passage of a current through the oscillograph thus recording the time of explosion. Resistance I5 insures that current will not fail to pass through the oscillograph in the event that a closed circuit occurs between leg wires 55' and 57' after the explosion.

What is claimed is:

1. A method for ascertaining the time of firing of an explosion initiated by an electric explosion initiator which comprises impressing an electrical potential between the firing circuit of said initiator and a point normally insulated from said firing circuit but in the locus of explosion, firing said initiator, and ascertaining the time at which a current passes by virtue of said potential.

2. Apparatus for ascertaining time of an explosion, comprising an electric explosion initiator, a firing circuit extending to said initiator, an auxiliary circuit including a portion of said firing circuit and also including a conductive path extending to a; point normally insulated from the firing circuit but in the locus of the explosion, whereby said auxiliary circuit is normally open-circuited but current is caused to flow therein by the explosion, and current-responsive recording means included in said auxiliary circuit to respond to current flow therein and to indicate the time of the explosion.

3. Apparatus according to claim 2, wherein said conductive path comprises a connection extending to a point within the explosion initiator but normally insulated from the firing circuit.

4. Apparatus according to claim 3, wherein the explosion initiator is provided with a metal shell normally insulated from the firing circuit, and said conductive path comprises a connection extending to said metal shell.

5. Apparatus according to claim 2, wherein said conductive path extends through the ground.

6. Apparatus according to claim 2, including a single source of voltage, and means for connecting said source both in said firing circuit and in said auxiliary circuit.

'7. Apparatus according to claim 6, including means for connecting said source first in said auxiliary circuit and then in said firing circuit.

DAVID M. McFARLAND.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,046,843 Prescott et al July '7, 1936 2,331,623 Parr, Jr Oct. 12, 1943 2,331,627 Petty Oct. 12, 1943 2,400,260 Miller May 14, 1946 

